Emergent feed-forward and isohydric responses to drought stress and atmospheric aridity: Insights and physical intuition from a time-dependent soil and plant hydraulic model

Drought and heat waves have synergistic effects on mortality as plants experience both supply and demand-side water stress. Yet which of these stresses most strongly down-regulates stomatal conductance ( g _s ), and whether patterns of regulation represent biological strategies or are imposed on the plant, remain at issue.

Dynamic losses of soil hydraulic conductivity are modeled in a soil domain fed by a steady maximum water flux from a deep-water source. A root-uptake plane extracts water from the soil in a two-node (root and leaf) plant model, with g _s a function of leaf water potential.

Multi-day simulations of a drought and heat wave experiment reproduce observations of anisohydric to isohydric transitions, driven by the balance of soil supply and atmospheric demand β. Feedforward control of transpiration E emerges from steep declines in soil hydraulic diffusivity that confine diurnal variation in moisture gradients and water discharge/recharge cycles to a shallow region at the root plane that thins in response to β.

Empirical models of VPD sensitivity are compared to the full model to provide mechanistic insight into empirical model parameters. Apparent responses of g _s to VPD emerge from plant, soil and atmospheric feedbacks that are both time and scale dependent.